Basically, two different types of electric home heating radiator are known. Firstly, electrical convection heaters, in which the ambient air to be heated is in direct contact with an electric heating resistance. These widely used electric convection heaters have the drawback of generating a strong movement of ambient air due to the thermal gradient created, causing discomfort to the occupants of the room concerned. This problem is partly solved by another type of radiator, called radiant heaters, operating by radiation.
Radiators using a heat transfer fluid are also known, in which said fluid, generally oil, is heated by an electric heating element and passes through a heating body, where the heat is transferred to the ambient air by natural convection. Due to the presence of the heating body, of which the heat exchange area is relatively large, the temperature gradient with the ambient air is reduced, so that the air movements by natural convection in the room concerned are limited.
Among these heat transfer fluid radiators, radiators in which the fluid operates in single-phase conditions are first distinguished. In these radiators, said fluid remains in the liquid state. In this case, the heat transfer fluid is heated in contact with an electric heating body, becomes less dense and rises inside the heating body. During its upward movement, the heat transfer fluid gives up part of the heat to the ambient air through the wall of the heating body, and commensurately cools. The fluid thus cooled, becoming denser, and therefore heavier, falls back by gravity to the lowermost part of the radiator. To ensure satisfactory operation of this type of radiator, it is therefore necessary to have a minimum temperature difference between the rising (hot) fluid and the descending (cold) fluid, which is directly dependent on the pressure losses of the fluid caused by its circulation. Accordingly, with this type of radiator, a nonuniform temperature distribution is observed in the wall of the heating body, which affects the efficiency of the radiator. Moreover, this type of operation can give rise to hotter spots on the surface of the apparatus, which are hazardous and also incompatible with the prevailing safety standards.
In order to overcome these drawbacks, document GB-A-2 099 980, for example, proposes a radiator using a heat transfer fluid operating in phase change conditions, in particular liquid/vapor conditions. Such a radiator operates as follows: the liquid heat transfer fluid rests by gravity in the lowermost part of the radiator traversed by a heating element, consisting of a fluid at elevated temperature, and passing through the base of said radiator in a sealed manner.
Under the effect of the heat, the heat transfer fluid is vaporized, said vapor thereby rising in the internal structure of the radiator, particularly at the level of the heating body, where the heat transfer occurs. As a corollary, since the temperature of the walls of said heating body is lower than that of the vapor, the latter condenses. The condensate thus formed is in liquid form, and returns to the lowermost part of the radiator by simple gravity.
This heat transfer mode, by phase change, and directly involving the latent heat of condensation, ensures a virtually uniform wall temperature of the heating body, accordingly constituting a very clear improvement over the heat transfer fluid radiators operating in single-phase conditions. This is because this transfer temperature is very close to the saturation vapor temperature of the heat transfer fluid owing to the much higher heat transfer coefficient in condensation than by natural convection from the outer side, that is the ambient air side. This achieves a substantial gain in the variation of the air temperature.
However, the heat source which raises the temperature of the heat transfer fluid proves to be relatively difficult to control, both in time and in space. Furthermore, it is observed that if the heat transfer fluid vaporization rate is too high, the vapor thereby generated entrains drops of heat transfer fluid, disturbing the satisfactory operation of the radiator.
Moreover, with such phase change radiators, the problem also arises of noise during startup. This noise is generated by the pressure waves during the collapse of the vapor bubbles in the subcooled liquid. Depending on the fluid used and the quantity of liquid fluid introduced into the radiator body, this noise generation may vary. In fact, this acoustic pollution may prove disturbing, or even prohibitive, for a number of applications, such as in particular hospital rooms, rest homes, retirement homes, or even simply bedrooms.
The present invention is precisely aimed to overcome these drawbacks, and in particular to propose a phase change radiator, that is both energy efficient and little or not noisy during its startup phase.